Chemistry Reference
In-Depth Information
E
2
1
E
1
1
consequence the
E
1
/
2
(
=
/
2
−
/
2
)
is the highest in the series.
[Cu
II
L
9
[Cu
II
L
9
)
•
]
+
[Cu
II
L
9
)
••
]
2
+
(
)
]
(
(
(8.17)
NIR spectroscopy reveals a remarkable feature for [Cu
II
Hx
L
8
)
•
]
+
: it exhibits a high intensity band at
1750 nm attributed to a phenolate - phenoxyl charge transfer.
115
(
Recently, the X-ray crystal structure of
this complex was solved.
118
In contrast to the results of X-ray absorption spectroscopy, the copper atom
was found to be in the (
III) oxidation state, whereas the ligand remains on its closed shell phenolate form.
These results have been rationalized by considering the existence of the following temperature dependent
equilibrium in solution (Equation 8.18):
+
[Cu
II
]
+
Hx
L
8
)
•
]
+
[Cu
III
Hx
L
8
)
(
(
(8.18)
Several diradical complexes were obtained from salen ligands. The electronic structures of [Cu
II
Hx
L
8
)
••
]
2
+
(
and [Cu
II
(L
9
)
••
]
2
+
compare well with that of [Cu
II
Et
L
2
OMe
)
••
]
2
+
. In contrast, the EPR spectrum of
(
[Cu
II
Et
L
8
)
••
]
2
+
(
is the superposition of two spectra, one typical of an organic radical (narrow signal at g
=
2.005), and the other corresponding to a mononuclear copper(II) complex, thus showing that the ethyl
linker is too flexible to maintain both phenoxyl moieties coordinated to the metal ion.
116
Copper(II) complexes of reduced salen ligands are much more readily oxidizable than the correspond-
ing “true” salen, the first electron transfer being for example observed at 0.02 V for [Cu
II
Et
L
10
)
],
119
(
and their copper(II) - phenoxyl complexes exhibit UV-Vis spectra more typical with bands at
≈
410 and
600 nm.
119 - 122
Hx
L
10
)
•
]
+
stoichiometrically oxidizes benzyl alcohol
in the absence of base. Despite the fact that this complex is a weaker oxidizer than [Cu
II
From a reactivity point of view, [Cu
II
(
Hx
L
8
)
•
]
+
,an
acceleration of the rate of reaction is observed and attributed to substrate preorganization upon binding.
123
On the other hand, [Cu
II
(
Et
L
10
)
•
]
+
oxidizes several unactivated primary alcohols into aldehydes in the
presence of base with turnover numbers up to 32.
119
[Cu
II
(
Ph
L
12
)
] is a particular salen complex in the sense that the copper resides in a square pyramidal
geometry as in GO. Unlike GO, which has a four-atom donor set, leaving the fifth position vacant for
substrate binding, no position is available in [Cu
II
(
Ph
L
12
(
)
] for substrate binding and subsequent catalytic
activity. Nevertheless, this complex oxidizes benzyl alcoholate in the presence of base.
124,125
This paradox
is explained by a pentacoordinated copper(II) - radical
hexacoordinated copper(III) - phenolate conver-
sion induced by coordination of the alcoholate substrate. Catalysis then proceeds by subsequent electron
transfer to the metal.
A
→
interest,
126 - 128
series
of
phenol - imidazole
copper
complexes
has
attracted
a
lot
of
as
[Cu
II
(L
13
Ph
(L
13
Ph
)
•
]
+
)
was
the
first
phenoxyl
radical
coordinated
to
a
copper(II)
ion
that
could
A emphasizes the C
=
be crystallized. In its X-ray crystal structure the C-O distance of 1.264(5)
O
character of this bond (it is 1.322(5) A in the corresponding bis-phenolate complex [Cu
II
(L
13
Ph
)
2
]). As
expected, the metal - ligand bonds in [Cu
II
(L
13
Ph
(L
13
Ph
)
•
]
+
are significantly longer than in [Cu
II
(L
13
Ph
)
)
2
].
All
these
bond
lengths
compared
well
with
those
obtained
in
the
chromium(III) - radical
complex
[Cr
III
(L
3
tBuOMe
)
•
]
+
.
The first functional model of GO involving an
o
-iminobenzosemiquinone was obtained from H
2
L
15
S
.
129
Mixing this tridentate ligand with copper(II) in the presence of triethylamine and air affords the dinuclear
copper(II) - phenoxyl complex [Cu
II
2
(L
15
S
)
••
2
], which is characterized by an absorption band at 404 nm
(8000 M
−
1
cm
−
1
and no signal in its EPR spectrum. Under similar conditions, by replacing H
2
L
15
S
by H
3
AP
L
15
AP
, the diamagnetic mononuclear copper(II) - phenoxyl complex [Cu
II
)
ISQ
L
15
AP
)
•
(NEt
3
)
]was
obtained and structurally characterized
130
(Figure 8.18; a similar complex involving two pyridines instead
of the triethylamine ligand has been reported previously).
131
(
The magnetic coupling between the metal and
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